But what makes good side characters?
There's no single, definitive answer to this question. However, after having finished both Buffy and its spinoff Angel, two crucially important things the show does well stood out: character growth and character agency. They seem simple, yet can be hard for writers to balance in with everything else they have to juggle in making their stories work.
We're already juggling the protagonist's character arc, the themes we want to implement, plotting for each individual episode, the overarching plot we wanna tell, and studio interference, and now we have to worry about side characters, too?!
We'll start with character growth. Most television shows have their side characters remain essentially the same until a certain episode specifically focuses on a specific side character. Said episode either reveals more about them or changes something about them. These episodes are great for making sure the side characters aren't forgotten, as it can be difficult to pay attention to them during the average episode of the series.
If you go through the list of Buffy and Angel episodes, however, you'd be surprised to see how few episodes are dedicated specifically to a certain side character. Yes, at specific points of different story threads certain characters might be extra active (more on that in a moment). In terms of episodes focused entirely on a single character, though, there are very few.
Quite simply, they don't need those episodes.
What Buffy and Angel do so well is weave arcs for their side characters into the plot of almost every single episode. There's no need to slow down the main story thread of a given season to focus an episode entirely on Willow, Xander, or any other side character when they are constantly facing their own challenges and growing throughout the series, often in ways that tie nicely into the main plot.
To get at what I mean in detail, let's look at Willow.
This is my favorite scene from How I Met Your Mother!
Willow starts out the series as a mousy nerd who lacks confidence and is really only good with computers. To say her character changes drastically would be an understatement. However, what's important to note is that the change doesn't happen over the course of a few Willow-focused filler episodes. Instead, we see her gradually change.
By the second season she's a little more confident and helping Giles with research. By the third season she's started on a path to become a witch and has come out of her shell enough to date Oz. By the fourth she's gained some skill as a witch and is dating a woman, Tara, with whom she has a romantic relationship similar in dynamic to her earlier friendship with Buffy, except her role is reversed: she became confident enough that she is the Buffy to Tara's past Willow. By the fifth she is one of the strongest members of the group, managing to actually hurt Glory, while also helping take care of Dawn. By the sixth she starts to abuse her powers, eventually leading to magic addiction, which she gets under control until Tara's death. In the seventh she is recovering from her magic addiction and suffering remorse for all she did as Dark Willow, reluctant to use her powers even when necessary.
These changes would've been impossible to organically implement if most episodes of the series didn't give at least a little attention to Willow. Throughout the whole series we see her growing through the challenges she faces and the choices she makes in response to those challenges, growing ever so slightly each time.
Or growing really quickly in the case of some characters.
This is true for every member of the main cast in both Buffy and Angel, especially those who stick around for the majority of the series.
But none of that growth would've been possible without the characters always having something important to do throughout each season. That brings us to character agency. As I mentioned in my post about the problem with the Canto Bight story thread in The Last Jedi, character agency "means how much a character's choices and actions affect the plot. Not in the generic, big picture summary of a character (Poe Dameron fights the First Order as a Resistance pilot) but in the minute-to-minute minutia of the story (everything Dameron actually does on screen)."
So, Buffy being a Vampire Slayer has nothing to do with character agency. However, every time we see her slay a vampire (or slay some other sort of demon, or choose to use mercy instead, or protect a demon being unfairly hurt, or even doing something innocuous that later has serious consequences) she is exercising character agency because her actions affect what's going on around her.
What's great about this series is that every character has agency. Of the core group, Giles uses his experience with the supernatural and research abilities, Willow uses her computer and magic skills, and Xander uses his humor and willingness to fight even when he doesn't have magic powers. Hell, at the end of season six he literally saves the world. How's that for agency?
Seriously, this friggin doofus accomplishes more in two minutes than 99% of any side character ever will.
It's not just the long-term cast, either. Take Tara, who shows up midway through season four and dies toward the end of season six. It'd be easy to have her just be nice to everyone and leave it at that, but throughout the series she both changes and has agency. Change-wise, she begins as a meek, timid character, as mentioned above. However, she gradually becomes more sure of herself, even becoming a surrogate parent for Dawn alongside Willow. When Willow starts dabbling in dark magic she stands up to Willow, even dumping her when the line is crossed. There's no way the Tara of season four would've been able to stand up for herself like that.
In terms of agency, Tara helps out with the magic duties of the group. While Willow surpasses her talent-wise, her experience with magic allows her to often be a guide of sorts in Willow's growth as a witch. It doesn't stop with her magic skills, however: she offers emotional support like no one else, often being the most nurturing person on the show; it's no coincidence she's the first one Buffy tells about her affair with Spike. She also exercises agency in helping Dawn as a surrogate parent, as well as directing Willow in her path to recovery by breaking up with her.
Basically, Tara both helps drive the plot and grows in the process.
I'm just posting this picture to slyly brag about the time I met Amber Benson,
the actress who played Tara (we share a birthday!).
I've talked mostly about Buffy, but let's look at a character from Angel. It'd be easy to trace the change characters like Wesley or Cordelia go through, but let's take a look at someone who joins later and doesn't have as distinct a role as everyone else: Lorne.
Lorne, the art and music loving demon with a seemingly infinite amount of personality, starts out as the owner of a nightclub who is neutral in the battle of good versus evil. He's more concerned with his nightclub than fighting the good fight. When Cordelia is taken to his home dimension of Pylea, however, he reluctantly joins, thereafter helping Angel's team whenever he can. Eventually he becomes a permanent member of the cast, helping without complaint until the fifth season, where he becomes the character most uncomfortable with the way the team has changed while running Wolfram & Hart. After being instructed by Angel to kill Lindsey, he informs Angel he's leaving afterward because of his disdain for killing.
Lorne's growth as a character is connected to everything he does for the team. The most obvious way he contributes is his ability as an empath demon to read the aura and immediate future of those around him. But he did so much more than that, too. He was the one character who never let his anger or despair make him an angsty doofus, and he often acts as the moral conscience of the group, especially during season five. The emotional labor he put in to always be supportive of his team was unrivaled by anyone else. Related to this, he was second only to Cordelia in helping Angel raise baby Connor. He also often figured out things the rest of the team couldn't, such as figuring out when Wolfram & Hart bugged the hotel.
Long story short, even Lorne, who was the last main character to join the cast and didn't actually fight, exercised agency in his contributions to the team and grew in the process.
All while also always having a cocktail at the ready.
So there you have it. While these are by no means the only two criteria for good side characters, they are quite important. Making sure your characters contribute gives the audience a reason to root for these characters, while making sure they change helps keep things fresh. Together, this powerful combination can make any character great...
...except maybe Connor.
Thanks for reading!